1. Field of the Invention
The present invention is related to an apparatus for separating stages and reducing thrust when a lower stage rocket motor of a multi-stage missile system is detached from an upper stage rocket or payload. More particularly, the present invention is related to an apparatus for separating a lower stage rocket motor from an upper stage without producing foreign object debris and for reducing the forward thrust of the lower stage rocket motor.
2. Technical Background
The use of multiple stage rocket motors has long been known as an effective means of increasing propulsion efficiency in a missile system. In a multi-stage missile system, an upper stage carrying a payload sits atop one or more lower stage rocket motors. Each lower stage rocket motor contains propellant within a case. The missile's propulsion system is typically initiated by igniting the lowest stage rocket motor. As the propellant burns, the combustion products are rapidly expelled out the aft end of the motor to provide thrust to the missile system.
Upon burnout of the motor or at some other desired time, stage separation and thrust reduction of the separated stage may be effected. Stage separation includes detaching the lowest stage rocket motor from the remainder of the missile. Following detachment, the lowest stage rocket motor is physically isolated from contact with the upper stage. Isolation is generally promoted by implementing some form of thrust reduction in the lower stage motor. Separating the unneeded stage from the missile typically improves propulsion efficiency by reducing the mass that rocket motors in subsequent stages must propel. If the next higher stage contains a rocket motor, that stage then becomes the current lower stage rocket motor. This new lower stage rocket motor may then be ignited to provide further propulsion to the missile. In this way, successive stages of rocket motors propel the payload toward its destination.
Multiple rocket motors may also be employed to increase the stealth capabilities of a missile system. Upon burnout of an initial stage motor, the spent motor may be detached. The pay-load may then coast a predetermined distance before ignition of the next stage, thereby reducing the risk of detection by heat-seeking missiles or other missile surveillance equipment.
Typical detachment mechanisms for releasing a rocket motor include a small explosive device which shears bolts or other retaining systems to release the lower stage rocket motor structure from the adjacent upper stage. Following detachment, the unneeded rocket motor is free to fall to the earth while the missile continues along its trajectory.
One of the most serious deficiencies of prior art systems for detaching lower stage rocket motors is that many such systems release debris into the air during the detachment process. Such debris, typically termed "foreign object debris," may include washers, bolt fragments and clamps utilized in the attachment of the lower stage rocket motor to the missile upper stage, as well as blow-out plugs, retainers and other hardware used to effect venting of the combustion chamber of the lower stage rocket motor. This debris poses grave risks to aircraft maneuvering near the missile.
Because many multi-stage missiles are designed to be launched from aircraft or otherwise utilized in areas where air-craft are deployed, aircraft may often be flying near such missiles during stage separation. Any collision between rocket motor debris and an aircraft is obviously undesirable. Debris from a separated rocket motor which is sucked into the jet engine of an airplane may severely damage the engine. Such engine damage may cause a loss of power to the plane, resulting in possible loss of the aircraft.
It is generally not feasible for pilots to avoid foreign object debris released during stage separation of missile systems. Even though the flight paths of multi-stage missiles are carefully planned, the path of expelled debris is difficult to control because individual pieces of debris may have shapes that are aerodynamically unpredictable. In addition, the small size of the debris and the high closure velocities at which an air-craft may encounter the debris generally make it impossible for pilots to visually detect and avoid oncoming debris.
Another serious deficiency of prior art designs relates to the manner in which they pursue separation of the upper stage from the lower stage rocket motor after detachment. Separation must be accomplished while addressing two primary concerns. First, the upper stage should be protected against exposure to combustion products from the lower stage rocket motor. In recognition of this concern, some conventional designs separate stages only after the lower stage rocket motor has burned out. A drawback of this approach is that optimal stage separation timing may be dictated by factors other than the amount of fuel left in the lower stage rocket motor. Such factors may include the risk posed by nearby heat-seeking missiles or the danger that continued thrust from the lower stage rocket motor will drive the missile off course or cause it to exceed the aerodynamic or structural limits of the missile. Such factors become increasingly important in missile systems designed to provide a high degree of mission flexibility.
A second principal concern to be addressed during separation is that a collision between the detached lower stage and the upper stage be avoided. Immediately following lower stage rocket motor detachment, the lower stage rocket motor and the upper stage are moving at essentially the same velocity. However, differences in the air resistance of these two bodies, possibly combined with post-separation thrust of the lower stage rocket motor, may cause a collision between the detached lower stage rocket motor and the missile upper stage. Such a collision could detrimentally alter the course of the missile. A collision may also damage fins or other portions of the upper stage.
Some conventional designs attempt to achieve separation by merely relying on wind resistance to slow the flight of the spent rocket motor case. In many cases, however, wind resistance alone acts too slowly to be effective. Also, reliance upon wind resistance is not possible outside the atmosphere.
Other designs utilize a drogue parachute or other aerodynamic drag-inducing device to slow the flight of the detached stage. Such designs are generally incapable of imparting sufficient drag to the detached stage to ensure safe and efficient stage separation. Also, many such designs are too expensive and complex to incorporate in many applications.
Simply firing an upper stage rocket motor to carry the missile away from the lower stage rocket motor is not always an adequate solution. The upper stage motor cannot begin firing before lower stage detachment and will not reach full power until some tens of milliseconds after ignition. The lower stage rocket motor may travel into the upper stage during this period. Furthermore, when the last rocket motor is separated from the pay-load, it is not always possible to fly the pay-load away from the detached stage. Some alternative measure is therefore needed to reduce or eliminate the risk of the lower stage rocket motor colliding with the upper stage after detachment.
From the foregoing, it will be appreciated that it would be an advancement in the art to provide a missile stage separation apparatus which does not produce foreign object debris.
It would also be an advancement in the art to provide such an apparatus that facilitates rapid separation of the lower stage rocket motor from the missile upper stage, thereby preventing the stages from colliding or otherwise adversely affecting the trajectory of the missile upper stage.
Such an apparatus is disclosed and claimed herein.